The present invention relates to an improved pneumatic tire and tread therefore, and more specifically to the contact of the tread with the ground during dynamic operating conditions.
The prior art incorporates the concept that the width of a tire footprint is designed to be load dependent. Moreover, as discussed in U.S. Pat. No. 3,782,438 (""438) and U.S. Pat. No. 3,949,797 (""797), when the tread of a pneumatic tire runs in a straight line, resistance to rolling increases as a function of the speed of travel. This resistance increases very rapidly (essentially exponentially) with increasing speed. The result is as the speed of tire travel increases, it is important that the effective width of the tire footprint., i.e. the tread on the ground, be as narrow as possible to minimize the dissipation of energy due to cyclic deformation of the tread. Cyclic deformation of the tread rubber causes heating of the pneumatic tire and the tread, in particular. By reducing frictional resistance, fuel consumption related to rolling resistance is reduced.
Further as discussed in the (""438) and (""797) patents, the phenomenon known as xe2x80x9caquaplaningxe2x80x9d or xe2x80x9chydroplaningxe2x80x9d occurs on a road surface covered with a layer of liquid, and specifically water, in which, the water cannot be evacuated fast enough from under the wheels of a vehicle traveling at a high speed by the grooves or designs in the tread of the pneumatic tires. As a consequence, the tires of the vehicle are lifted or separated from the ground and are supported only by the layer of liquid. This in turn, results in practically complete loss of adherence of the tire to the ground and consequently, the loss of steering or directional control of the vehicle and the risk of skidding, swerving, and serious accidents. The critical speed at which such a dangerous phenomenon may occur is a function of various parameters or factors such as the inflation pressure of the pneumatic tire, the vehicle load on the tire, the depth of the tread design of each pneumatic tire, the effective width of running contact (the tire footprint) with the road, etc. The critical speed is an inverse function of the effective width of tire footprint with the road such that a narrower footprint (a narrower tread) on the ground results in a higher critical speed, which is preferable for decreasing the risk of the xe2x80x9caquaplaningxe2x80x9d phenomenon. It can be stated, however, if the effective running contact or footprint, i.e. the tread is narrow, the pressure in the area of contact with the ground is generally higher.
In addition, a narrower tread may be less stable and adversely effect the vehicle behavior during turning. For example, while turning a vehicle through a curve at a relatively high speed, the centrifugal force and possibly other passive forces of inertia or the lateral or transverse reaction acting on each tire, tend to make the tires tilt to the side, so that a portion (radially internal with respect to turning) of the tread of each pneumatic tire, which is initially in contact with the ground, rises and separates from the ground. As the speed increases, the tread adheres to the ground by smaller and smaller portions of the footprint (radially exterior with respect to turning) which increases the risk of skidding. Consequently, a narrow tread, i.e., a small effective width of running contact or small footprint, is generally undesirable to the safety of travel during turning.
It is therefore seen that the requirements or conditions for maintaining an optimum route or optimum behavior of an automobile vehicle are contradictory and relatively incompatible for straight-line travel and turning, since in straight-line travel it is important that the effective width of running contact with the ground or the tread of each pneumatic tire tread be as narrow as possible, while it must, in contrast, be as wide as possible for turning motion.
Moreover, during braking, a wider contact width of the tire footprint is preferred so as to increase frictional force and thus yield a shorter braking distance.
An example of a variable tread design is disclosed in U.S. Pat. No. 4,823,855 (""855) to Goergen et al., having a common assignee with the present invention. The ""855 Patent discloses xe2x80x9ca tire according to the invention that due to its cross-sectional shape and preferred tread geometry, the tire will operate to provide preferred performance characteristics at different operating loads and in different operating conditions. For example, when the tire is mounted upon a small all-purpose vehicle and operated on a paved roadway at everyday commuting loads, which are far less than the tires rated load, the central portion CP of the tread is almost exclusively in contact with the roadway. The higher net-to-gross ratio of this central portion of the tread provides good tread wear and on-highway handling. When the tire is operated under higher loads and/or off-the-road the laterally outer zones OP1, OP2 of the tread also come into operation to provide greater traction in mud or sand and improve off-highway handling. The varying stiffness of the block elements across the tread also contributes to the performance characteristics of the tire under varying operating conditions.xe2x80x9d
The present invention relates to a radial ply tire with a carcass having a crown region, two sidewalls extending from a first end of the sidewalls radially inward from the surface of the crown region, and two beads disposed at a second end of the sidewalls and one or more plies extending through the crown region and between the beads and wrapped thereabout. A belt structure overlies the crown region of the carcass and a tread overlies the belt structure and is secured to the sidewalls in the shoulder portions. The tread is subdivided transversely into a central portion having central ribs, shoulder portions having shoulder ribs and intermediate portions having intermediate ribs between the central portion and the shoulder portions. The tread has first and second intermediate grooves between the central ribs and the intermediate ribs. First and second shoulder decoupling grooves are disposed between the intermediate ribs and the shoulder ribs. The tread may or may not have a central groove. The first and second shoulder decoupling grooves having a depth (D1) being 40% to 70% the depth (D2) of the first and second intermediate grooves whereby the footprint width of the tread surface can expand from first width (NLFPW) under normal load to a second width (HLFPW) under heavy load wherein the second width is 10% to 40% greater than the first width.